Electrodialysis



sept? 2o, 1932.

R. A. GoR'rN'ER E'r AL ELEcTRoDILYsIs Filed Aug. 2o, '1928 PatentedSept. 20, 1932 UNITED STATES PATENT OFFICE ROSS AIKEN GORTNER, 0F ST.PAUL, AND WALTER F. HOFFMAN, 0F CLOQUET, MINNE- SOTA, ASSIGNORS T0NORTHWEST PAPER COMPANY,- OF CLOQUET, MINNESOTA, A

CORPORATION 0F MINNESOTA unaccmonamrsrs Application 1ed August 20, 1928.Serial No. 300,662.

The present invention relates to electrodialysis and to method and meansfor increasing the time and current efficiency of the process. It hasparticular reference to the Ifemoval of inorganic matter and othersubstances capable of migration, or which can be caused to migrate,under the influence of the electric current from liquid or solidmaterial, such as the impurities in lchemical wood pulp.

In a copending application Ser. No. 54,003, filed Sept. 2, 1925, by R.A. Gortner and W. F. Hoffman, of which this application is acontinuation in part, there is described a process for treating woodpulp and like libers to remove coloring matter, mineral matter and otherimpurities by subjecting a slurry of the pulp to the action of anelectric current in a diaphragm cell. The impurities are moved by thecurrent from their former association with the pulp, into the liquid andfrom the liquid through'the diaphragms and away from the pulp. Theelectrical charge may ybe natural, or it may be induced in theimpurities by the addition of certain materials.

In carrying out said process it haswbeen the usual practice to employ aslurry of low concentration of Tiber with the intent to limit thequantity of matter to be removed from a single batch, and hence toreduce the time required.

The present invention is based upon the observation that when a moreconcentrated slurry of pulp is employed, a unit quantity of liber can bepurified in less time with less power consumption per unit of fibertreated. A study of the condition of operation has revealed that theratio of fiber to suspending liquid is important, and that the time andcurrent efficiencies increase with increase in concentration of slurrylwithin the workable ranges of the process. I v

Accordingly, one object of the present invention is to control theconsistency of any stock which is subjected to electrodialysis so as ltoobtain increased efficiency.

Another-object of the invention is the provision of a process forefficiently purifying may or may not be a source of the material to beso removed.

Still another object is to improve the dialysis processby other meanswhich produce the same eect as the addition of liber or pulp.

In order to explaink theprinciples of the invention We have chosen toillustrate it by reference to wood pulp as it is dialyzed in the cellshown in the accompanying drawing. The process is not limited to suchmaterial nor apparatus as Will be clear from the following descriptionand explanation of what is believed to be the underlying principleinvolved.

In the drawing the cell is illustrated in diagrammatic form. A suitablecontainer l0 of box like structure may be constructed of wood and madewatertight in any suitable manner, such as by coating withtar or pitch.Atypical cell which has been employed, but which'may be modifiedin its ydimensions without departing from the invention, is l0 inches long, 6inches Wide, and

5 inches deep. Larger cellsemploying 2000 pounds of slurry have beenemployed., Partitions 11 and 12 are provided which are permeable tosubstances which it is desired to move out of the main chamber, andimpermeable to substances desirably retained or animal tissue whichpermit ions to pass and which retains such materials as colloids.

The partitions divide the box into three compartments, the maincompartment 13, the anode compartment. 14, and the cathode compartment15. The size of the compartment is immaterial, but for efficiency, smallelectrode compartments are preferred. Suitable electrodes 1o and 17 ofcarbon may be used such as graphite plates. 'Carbon is the preferredmaterial for the anode as it is sub- `Ject to corrosion, but the cathodemay be iron, 'copper or 'other metal which is not attacked as cathode.Any cell involving the essential features of the exemplary cell abovedescribed may be used in the process of this invention.y

In use, a slurry of pulp is placed in the middle compartment 13. Acurrent of electricity is passed through the cell by applying a voltagetothe electrodes. A voltage of about 200 has produced a current of aboutone ampere in the cell above described. The current, however, variesduring operation, because the resistance of the cell increases asimpurities are removed. When certa-in pulps high in electrolyte contentare employed an initial current as great as ten'amperes may be observed,in which case the applied voltage may be reduced merely to preventoverheating. During the operation the impurities are caused to migrateinto the electrode compartments, and in order to reduce the tendency ofthese impurities to fiow back through the partitions they are removedfrom the electrode compartment by providing intermittent or a constantreplacement of water therein.

It will be clear from the foregoing description of the apparatus thattoo concentrated a slurry will form a mechanical ba-rrier to themigration of the impurities through the solution and it will alsominimize the water channels through which migration may take w` place.We have found that an 8% slurry of a commercial sulphite spruce pulp isclosel to the upper limit of concentration. When a more dilute slurry isemployed, such as a 3% slurry greater current and a longer time will berequired than when an 8% slurry is used, and this in spite of the factthat there i's only three-eighthsas much impurity to be removed.Accordingly, we believe that an 8% slurry lies between the upper andlower limits of concentration for chemical wood pulps.

We are unable to set by exact figures the lower limit, but believe thelimit to be determined by the manner in which current is proportionedbetween liquid and pulp. In a very thin slurry current is carriedlargely by the suspending liquid. In a 3% slurry the mass is mobile andmoves about to a disadvantage. An 8% slurry is thick and but slightlymobile. The condition approximates a close packing of the wet fibers.Such a condition is believed to establish a continuous fibrous pathacross the cell for conducting the current, and the fibrous path to beof greater conductivity than the water paths.

This belief is supported by the observation that ashless fibers, such asthose prepared by this process, when added to distilled water, or towater containing electrolytes, have the property of increasing theconductivity generally in proportion to the fiber concentration up to acertain limit. This limit of course depends upon the particularconditions and materials involved. The added fibers are by natureinsulating in character when dry, but when in combination as abovedescribed they appear to have a property of conductance. Whereas, onewould expect a less conductivity by adding an insulating solidnon-ionizing material to a conductive liquid, it is nevertheless a factthat a greater conductivity is found when pulp is added to water.Accordingly, in the cell with the high concentrations of fiber currentappears to be carried principally by the liquid associated with thefibers. The potential diHerence causing current to flow tears theimpurities apart, or away from their fixed condition with the fibers, sothat they may readily enter the liquid current-carrying channels forfree approach to the separating diaphragms.

It is not known whether the path of increased conductivity at the fibersis through the fiber or at the surface of the liber, or whether it isat-both, or elsewhere. Inasmuch as distilled water with pulp free fromelectrolyte has only ions of water present to carry electric charges, itis possible that a selective separation of the ions has been made by theaddition of pulp. This could be explained by the double layer theorycharacterizing many adsorptive and surface phenomena. According to thistheory positive and negative charges which normally neutralize eachother are separated into two layers in such a manner that each layerassumes a positive or negative charge indicating a disturbance of thenormal neutralization. In accordance with this theory it is possible tosuppose that with a proper concentration of pulp in water, the mixturesubjected to electrodialysis contains positive and negative channelsthrough the mass into which channels various charged impurities aredrawn and in which they migrate with rapidity and with high currentefficiency.

The time and electrical efficiency of the process of this invention asapplied to pulp is greater than when the dialyzing is carried out withconcentrations under the lower limit. For example, a 3% slurry operatingona given applied voltage has taken over a week to reach a given stageof completion, while an 8% slurry under the same conditions of appliedvoltage has attained the same stage in two days. Although the 8% slurrycarries a higher current, the power consumed per unit of pulp, is muchless than required for the 3% slurry, and the time is much shorter.

With too low a concentration of pulp we have determined that when theash content of pulp is reduced to 0.1% the ionic concentration resultingfrom such ash is insufficient to give a current capable of removing theash without anundue expenditure of time and electric energy. But whenthere is a so-calledsurface conductance across the cell caused by thepresence of sufficient pulp an increased current will flow. Suchincreased current causes rapid and eicient removal of the ash ,to muchlower concentrations and even permits rsubstantially complete removal.In other words, ionic conductance by the impurities to be removed is notrelied upon, but rather the so-called surface conductance along thefibers. It appears difficult to define the limits of concentration, butgenerally it is believed a sufiicient concentration is one at whichthere is an appreciable conductance through the mass after theelectrolyte content has been decreased to a point where there is noappreciable ionic conductance through the body of the liquid which liesin the'interstices of the fiber mass.

Although the invention has been described and explained with referenceto wood pulp it is not to be considered as limited to wood.

`pulp. It-has been carried out with other materials such as cinchonabark, from which quinine has been removed'by the process described in acopending. application of Anton Hogstad, J r. Ser. No. 95,203, filedMarch 17,

1926, nux vomica and other alkaloid-contain ing materials, and otherprocesses of electro-` dialysis of distinctive character. The inventiondoes not necessarily require that pulp or fibrous material presentduring the dialysis be the source o f material to be recovered, for

pulp will function according to this invention independently ofimpurities therein. In.

consequence, the invention contemplates that fiber, such as wood pulp,be added to any electrodialysis cell for ,the purpose of producing aneffective concentration of solid material therein to give all theadvantages herein set forth.

The present invention provides for the presence of sufiicient solidmaterial, such as the fiber of the wood pulp, which has the property ofimparting to the suspending liquid an electrical conductance distinctfrom and in addition to. that which is due to conducting ions in theliquid in the absence of such solid material. When it is attempted toremove the last portions of electrolyte from a solution (solid beingabsent) by the migratory influence of current flowing through the 4solution, itis clearly obvious to those who are informed on suchprocesses, that the current decreases as the electrolyte decreases, forthel reason that the strength of current depends in part upon the amountof electrolyte pres-i ent. But when an additional conductance isprovided by the presence of solid as described p in this invention theremoval of the last portions does not operate to reduce the current to asubstantially ineffective value. It has been found that the additionalcurrent is fully as effective, (if n ot more effective) in removing theelectrolytes, as the current due directly to such electrolytes.

We are aware of the fact that wood pulp has`a`high degree of dispersion,that is, that it provides a very large surface, upon which y surface webelieve there is a special conductance phenomenon. We are aware thatother substances having a high degree of dispersion exhibit a simllaryproperty of increasing conductance of liquids in which they are sus,

pended. For example, asbestos fibers, silica gel, precipitated and driedaluminum oxide, clay, carbon, etc. Other substances exhibit the propertybut nevertheless in a less pronounced degree. Powdered glass has aslight effect, and powdered quartz has an even less effect. Providingour theory is correct as to the so-called surface conductance along suchmaterials, any of the foregoing substances may be added to materialbeing electrodia-l'- lyzed in order to' increase the speed of migrationand to decrease the power consumption.

Another theory which can be offered to explainthe effect produced bytheaddition of pulp is `that the increased current which is shunted acrossthe liquid channel through the fibers creates an increased magneticfield in which the ions and charged particles in the liquid or thesolution are more readily moved by the applied potential at the elecandnot inconsistent with a theory of operation herein presented, andfurthermore in terms calculated todefinethe invention in its broadestscope including the range of equivalents which can be ascertained uponestablishing the nature of the forces involved in the process.

We claim:

1. The method of' removing impurities from chemical wood pulp whichcomprises subjecting an aqueous suspension of said pulp in anelectrodialysis apparatus to the action of an electric potential to forma current through thefmass, the pulp being present in sufficientquantity to provide towards the completion of dialysis a major portionof thel the vicinity of said confining means after pascurrent along afiber channel and a minor porsage therethrough.

tion in a water channel, permitting impuril In witness whereof we havehereunto afties to migrate away from the mass, refixed our signatures.

straining movement of the pulp, and restraining movement of the liquidaround electrodes from entry into the suspending liquid about the pulp.i

2. The method of removing f impurities from fibrous material whichcomprises subjecting an aqueous suspension of fiber in anelectrodialysis apparatus, to the action of an electric potential toform a current through the mass, the fiber being present in suiiicientquantity to proyide towards completion of dialysis amajor portion ofcurrent along a fiber channel and a minor portion in a water channel,permitting impurities to migrate away from the mass, restrainingmovement of the fiber, and restraining flow of liquid across asemi-permeable partition separating theisuspension from the electrodes.

3. In an electrodialysis process the method of increasing the speed anddecreasing the consumption of power necessary to move the last portionsof material which is caused to migrate in a liquid by the electriccurrent which comprises, adding fibrous material in suiiicient quantityto provide a current channel along the ber' having a conductance greaterthan the conductance of the liquid in the absencel of said fibers at atime near the end of the process.

4; In an electrodialysis process the method of increasing the speed anddecreasing the consumption of power necessary to move the last portionsof material which is caused to migrate in a liquid by the electriccurrent which comprises, adding material having a.

high degree of dispersion in, suiicient quantity to provide a currentchannel along such material having a conductance greater than theconductance of the liquid in the absence of such material at a time nearthe end of the process.

5. In an electrodialysis process the method of increasing the speed anddecreasing the consumption of rpower necessary to move the' lastportions of material whichis caused to migrate in a liquid by theelectric current' which comprises, adding material capable of producinga conductance along the surface of said material when immersed in theliquid.

6. The method of electrodialysis which comprises subjecting a confinedsuspension of fibrous material which provides a continuous ibrous pathtoan electrical potential whereby a current ows along said' continuousfibrous path, confining the mass by means permeable to passage of theresulting migrating units in the mass and impermeable to `fiow of liquidtherethrough, whereby said units are removed by the current from themass, and removing some or sald units from

